ZSFG Morning Report: Dactylitis ddx, IGRA’s, and a hint of NTM/TB

Thanks to Lily Kornbluth for presenting the case of a woman from China admitted with a swollen and painful finger which turned out to be mycobacterial dactylitis!

You’re probably asking yourself: Wait, aren’t those swollen digits just in psoriatic arthritis??? Do I have a framework or a DDX FOR DACTYLITIS??? Well, NOW YOU DO! Here’s a short list generated in report today with the help of Lisa Winston, Stephanie Cohen, Lily, and the #fabulous ZSFG resident crew!

As we narrowed in on the dx, a diagnostic test returned: POSITIVE QUANTIFERON GOLD! Lisa Winston reminded us about the power of the QTF-Gold and that it’s more specific than the PPD! Here’s a quick refresher and an elegant representation of the box/test….

QuantiFERON Gold or more technically since it just rolls off the tongue: Interferon-gamma release assays (IGRA’s):

Of note, ESAT-6 and CFP-10 are encoded by genes in the region of difference 1 (RD1) segment of the M. tuberculosis genome. These antigens are more specific for M. tuberculosis than the PPD because they are not shared with any BCG vaccine strains or most species of NTM.

HOWEVER: Two of the NTM that affect humans, Mycobacteriummarinum and Mycobacterium kansasii, contain gene sequences that encode for ESAT-6 or CFP-10. So infxn with either of these NTM’s=positive QTF test 😦

What are their test characteristics?

IGRA’s have specificity >95% for diagnosis of latent TB, especially great as unaffected by BCG vaccination. The sensitivity for the IGRA’s T-SPOT.TB and QFT-GIT are 90 and 80%, respectively. PPD sensitivity as reference is 80%. Of note, IGRA sensitivity decreases in patients with HIV!

Once the deep skin biopsy came back “swimming in AFB’s,” we discussed how to approach mycobacteria based on rapid or slow growth. Here is a simplified algorithm to consider…